The present invention relates to a heater for the pyrolysis of hydrocarbons and particularly to a method of operating a pyrolysis heater with reduced NOx emissions.
A pyrolysis heater may also be referred to as a pyrolysis furnace. A pyrolysis heater is any device for the pyrolysis or steam cracking of hydrocarbons.
The steam cracking or pyrolysis of hydrocarbons for the production of olefins is almost exclusively carried out in tubular coils placed in fired heaters. The pyrolysis process is considered to be the heart of an olefin plant and has significant influence on the economics of the overall plant.
The hydrocarbon feedstock may be any one of the wide variety of typical cracking feedstocks such as methane, ethane, propane, butane, mixtures of these gases, natural gas, naphthas, gas oils, etc. The product stream contains a variety of components, the concentration of which are dependent in part upon the feed selected. In the conventional pyrolysis process, vaporized feedstock is fed together with dilution steam to a tubular reactor located within the fired heater. The quantity of dilution steam required is dependent upon the feedstock selected; lighter feedstocks such as ethane require lower steam (0.2 lb./lb. feed), while heavier feedstocks such as naphtha and gas oils generally require steam/feed ratios of 0.5 to 1.0. The dilution steam has the dual function of lowering the partial pressure of the hydrocarbon and reducing the carbon laydown rate in the pyrolysis coils.
In a typical pyrolysis process, the steam/feed mixture is preheated to a temperature just below the onset of the cracking reaction, typically 650° C. This preheat occurs in the convection section of the heater. The mixture then passes to the radiant section where the pyrolysis reactions occur. Generally the residence time in the pyrolysis coil is in the range of 0.2 to 0.4 seconds and outlet temperatures for the reaction are on the order of 700° to 900° C. The reactions that result in the transformation of saturated hydrocarbons to olefins are highly endothermic thus requiring high levels of heat input. This heat input must occur at the elevated reaction temperatures. It is generally recognized in the industry that for most feedstocks, and especially for heavier feedstocks such as naphtha, shorter residence times will lead to higher selectivity to ethylene and propylene since secondary degradation reactions will be reduced. Further it is recognized that the lower the partial pressure of the hydrocarbon within the reaction environment, the higher the selectivity.
The flue gas temperatures in the radiant section of the fired heater are typically above 1,100° C. In a conventional design, approximately 32 to 40% of the heat fired as fuel into the heater is transferred into the coils in the radiant section. The balance of the heat is recovered in the convection section either as feed preheat or as steam generation. Given the limitation of small tube volume to achieve short residence times and the high temperatures of the process, heat transfer into the reaction tube is difficult. High heat fluxes are used and the operating tube metal temperatures are close to the mechanical limits for even exotic metallurgies. In most cases, tube metal temperatures limit the extent to which residence time can be reduced as a result of a combination of higher process temperatures required at the coil outlet and the reduced tube length (hence tube surface area) which results in higher flux and thus higher tube metal temperatures. The exotic metal reaction tubes located in the radiant section of the cracking heater represent a substantial portion of the cost of the heater so it is important that they be utilized fully. Utilization is defined as operating at as high and as uniform a heat flux and metal temperature as possible consistent with the design objectives of the heater. This will minimize the number and length of the tubes and the resulting total metal required for a given pyrolysis capacity.
In the majority of cracking furnaces, the heat is supplied by floor burners, also called hearth burners, that are installed in the floor of the firebox and fire vertically upward along the walls. Because of the characteristic flame shape from these burners, an uneven heat flux profile is created. The typical profile shows a peak flux near the center elevation of the firebox, with the top and bottom portions of the firebox remaining relatively cold. In select heaters, radiant wall burners are installed in the top part of the sidewalls to equalize the heat flux profile in the top portion of the heater. Improving the heat flux profile is complicated by NOx emission considerations.
Nitrogen oxides (NOx) are produced in essentially all combustion processes using air as the oxidant gas. NOx is produced primarily as nitric oxide (NO) in the hottest regions of the combustion zone. Some nitrogen dioxide (NO2) is also formed, but its concentration is generally a small percentage of the total NOx.
Nitrogen oxides are among the primary air pollutants emitted from combustion processes. NOx emissions have been identified as contributing to the degradation of the environment, particularly degradation of air quality, formation of smog (poor visibility) and acid rain. As a result, air quality standards are being imposed by various governmental agencies, which limit the amount of NOx gases that may be emitted into the atmosphere.
In addition, there is an inverse relationship between NOx and CO formation which further complicates emissions control. Combustion processes do not perfectly bring together the three T's (time, temperature, and turbulence) to achieve complete combustion, and some amount of CO generation is inevitable. Generally speaking, the higher the peak combustion temperature, the lower the CO generation. Unfortunately, the trend is just the reverse for NOx generation; the higher the combustion temperature, the greater the NOx generation. Therefore, emission control for industrial combustion sources must compromise between NOx and CO control.